Some wheel loaders and other work vehicles are commonly equipped with a so-called hydrostatic transmission (HST). As described in Japanese Laid-open Patent Application No. 2008-275012, in an HST-type work vehicle, a hydraulic pump is driven by an engine, and a travel hydraulic motor is driven by hydraulic fluid discharged from the hydraulic pump. The work vehicle is thereby made to travel. In such an HST-type work vehicle, the vehicle speed and traction force can be controlled by controlling the engine speed, the displacement of the hydraulic pump, the displacement of the travel hydraulic motor, and other factors.
The work vehicle is also provided with an engine controller, and the engine controller controls the output of the engine in accordance with a pre-set engine output torque curve. FIG. 13 is a view showing an example of the engine output torque curve. The engine output torque curve indicates the upper limit value of the torque that can be outputted by the engine in accordance with the engine speed. The engine controller controls an engine fuel injection device so that a target engine speed that is in accordance with a set throttle opening degree is obtained. The throttle opening degree is set in accordance with an operation amount (hereinafter referred to as “accelerator operation amount”) of an accelerator operation member operated by an operator. In FIG. 13, the solid line L100 indicates the engine output torque curve when the accelerator operation amount is 100%. This engine output torque curve corresponds to, e.g., the rated or maximum power output of the engine. The broken line L75 indicates the engine output torque curve when the accelerator operation amount is 75% in a high-load work phase. The engine controller controls the output of the engine so that the output torque of the engine is equal to or less than the engine output torque curve.